A parametric study has been conducted to determine the optimum physical properties of phase-change energy storage materials for solar air-heating systems. Simulation techniques are used to determine the system performance over the entire heating season. Variations of the solar fraction of the load with melting temperature, latent heat, load characteristics, and control strategy have been determined. Air-heating systems with a wide range of hot water and space heating loads have been examined. The effect of semicongruent melting of the phase change material on system performance has also been investigated. © 1978.